arm: Remove spurious .fnend directive

...a.k.a. Fix annoying build error

Some toolchains rightly object to the fact that once the acros are
expanded this results in multiple ".fnend"s without ".fnstart"s.

See: https://github.com/raspberrypi/linux/issues/4836



Signed-off-by: Phil Elwell <phil@raspberrypi.com>